Resonant DC/DC converters which make it possible to transform one voltage level into another are frequently implemented in high power-density and high-performance DC/DC conversion systems.
A simplified basic architecture of a resonant direct current—direct current converter (or DC/DC according to the corresponding acronym) of the “LLC series” type (i.e. comprising two inductive resistors 1, 2 and a capacitor 3 in series) is represented in FIG. 1.
At the input, two field effect power transistors 4, 5 of the MOSFET type are connected to a direct current source 6, forming a half-bridge with a first, so-called high-side transistor 4 connected to the potential terminal of the source 6, and a second, so-called low-side transistor 5 connected to the earth.
The resonant circuit 7 comprises in series the capacitor 3 and a first inductive resistor 1 which determines the resonance, and a second inductive resistor 2 of a transformer 8. At the output there are two rectifier diodes 9 and a filtering capacitor 10 which supplies a load resistor 11 with direct current.
The two power MOSFETs 4, 5 are switched in a complementary manner according to a duty cycle close to 50%, leaving a constant dead time in order to avoid a phenomenon of simultaneous conduction.
This known LLC converter functions according to a zero voltage switching mode of all the semiconductors 4, 5 in a wide range of charges, with improved EMC (electromagnetic compatibility) performance and a limited switching frequency.
However, because of the substantial pulse currents on the secondary of the transformer 8 and high currents on the primary, the use of an elementary converter of this type is limited to low or medium power levels. In fact, applications which require high powers and strong currents lead to iron losses and additional switching losses which reduce the global performance of the converters.
In order to produce high-power converters and eliminate these disadvantages, use is made of a multiphase architecture in which a plurality of identical elementary converters are connected at the input in parallel on a single source 6, and at the output in parallel on a single load 11 (parallel-parallel LLC multiphase converter), as shown clearly in FIG. 1, in order to share the total power amongst all the elementary converters and to distribute the currents better between the different power units in order to obtain good performance.
The current which circulates in the primary of the transformer 8 can be reduced, and the current constraints imposed on the MOSFET transistors 4, 5 are reduced and distributed between the different power units.
A well-known control method for the transistors 4, 5 of the half-bridges of the elementary converters of the multiphase converter consists of making the n phases function (n being equal to or more than 2) at a common switching frequency, with a phase shift Δφ of T/n (period number of units) between two adjacent elementary converters, in order to obtain an output current with fewer pulse transients.
However, this well-known control method is valid only in the hypothesis that all the elementary converters have exactly the same electrical characteristics, i.e. same first induction coil LR1, LR2, LR3 at the resonance of the first inductive resistor 1, same capacity CR1, CR2, CR3 at the resonance of the capacitor 3, same second induction coil LM1, LM2, LM3 of the second inductive resistor 2, and same switching parameters QH1, QH2, QH3; QL1, QL2, QL3 of the transistors 4, 5.
This means that a type of super-symmetry must be maintained between all the elementary converters.
In the event of malfunctioning, the slightest dissymmetry is liable to create enormous balancing problems, with the largest fraction of the current passing via a single power unit and leaving the other units functioning at a low output power, or even zero power.
For example, a difference of 5% between the first induction coil LR1 of the first inductive resistor 1 and another can introduce a current imbalance which can reach 90%.
Since each parameter of the electronic component has a certain tolerance (typically in the motor vehicle industry ±5% for a capacitor and ±10% for an inductive resistor), this well-known control method is in fact totally inefficient for ensuring the balancing of the currents between the different elementary converters, and therefore for maintaining acceptable global performance.
European patent application EP2299580 proposes a method and a device for controlling a multiphase resonant DC/DC converter in order to solve the problem of imbalance of the currents in elementary converters of the LLC type, without costly selection and matching of the components.
As represented schematically in FIG. 1, the method consists in particular of measuring the supply currents of the elementary converters (three being represented) by means of shunts 12, and of controlling the phase shifts Δφ1-2, Δφ2-3 between the control signals, with the same common frequency, of the MOSFET transistors 4, 5 of the half-bridges, such as to balance these supply currents.
However, this method is close to the conventional method, and computer simulations carried out by the inventive body have shown that this method was not optimum. Ways therefore remain for improvement of a control method of the same type, making it possible to eliminate the above-described disadvantages.